FIG. 1 illustrates a classical current fed inverter 116 connected to an RLC resonant load. The resonant load may be, for example, induction coil, Lcoil, that is used with an industrial electric induction furnace or heating device, in series with resistance element, R, that represents a magnetically coupled work load (for example, metal in a furnace or a metal gear placed in the induction coil) when ac current flows through the induction coil, and tank (resonant) capacitor, Ctank, connected in parallel with the induction coil and resistance element. The typical inverter is illustrated as a H-bridge with switching devices, S1 through S4 supplying current sequentially through switch pairs S1-S4 and S2-S3 to the resonant load during alternate electrical half-cycles. Regulated dc current is fed to the inverter through smoothing inductor, or choke, Lchoke, which current is supplied from a suitable source. In FIG. 1 a three-phase, full-wave, variable-voltage rectifier 112, formed from silicon controlled rectifiers, SCR1 through SCR6, or other types of phase-controlled electric switches, is used to supply the dc current. Input to the rectifier is from a suitable ac source, such as a 50 or 60 Hertz, three-phase utility supply (designated as lines A, B and C in the figure).
FIG. 2 graphically illustrates performance characteristics of the inverter shown in FIG. 1. Parameters are inverter output current magnitude, Iout(INV), in amperes, as a function of frequency; inverter output power magnitude, Pout(INV), in kilowatts, as a function of frequency; inverter output voltage magnitude, Vout(INV), in volts, as a function of frequency; maximum dc current, Imax, in amperes; maximum inverter output power, Pmax, in kilowatts; and maximum inverter output voltage, Vmax, in volts.
Rated (maximum) operating condition is defined by the intersection of the curves identified by operating line L2. Resonant operating condition is defined by the minimum values of inverter output voltage, current and power as defined by operating line L1. The inverter output voltage across the resonant load can be expressed by the formula:
            V      inv        =                  V        dc                              0.9          ·          cos                ⁢                                  ⁢        φ              ,
where Vinv is the output voltage of the inverter, Vdc is the supplied dc voltage, and φ is the phase shift between inverter output current and voltage.
The output power of the inverter (Pinv) is proportional to the square of the inverter voltage:Pinv≈Vinv2.
Consequently to increase power, the resonant load will operate off resonance, with increasing reduction in efficiency as the power level increases. To reduce inverter output power to a level lower than that at resonance, the dc output from the rectifier is reduced by phase control of the rectifier's switches.
It is one object of the present invention to provide power control of a current fed inverter while keeping the load at resonance, by means other than phase control of an input rectifier. Another object of the present invention is to minimize the size of reactive components used in the inverter.
Another object of the present invention is to provide uniform mixing of an electrically conductive material, such as a molten metal bath, placed within an induction furnace.
Another object of the present invention is to maximize the surface area of the molten bath that is exposed to ambient environment, particularly when that environment is substantially a vacuum, to promote outgassing by modulating the convex meniscus forming the surface area.